While Ziegler-Natta catalysts are a mainstay for polyolefin manufacture, single-site (metallocene and non-metallocene) catalysts represent the industry's future. These catalysts are often more reactive than Ziegler-Natta catalysts, and they produce polymers with improved physical properties. The improved properties include narrow molecular weight distribution, reduced low molecular weight extractables, enhanced incorporation of α-olefin comonomers, lower polymer density, controlled content and distribution of long-chain branching, and modified melt rheology and relaxation characteristics.
Recently, non-metallocene, single-site catalysts that incorporate a Group 3-10 transition metal and pi-bonded heterocyclic ligands that are isolobal with the cyclopentadienide anion have been described. Examples of such “Cp-like” ligands are boraaryl (see U.S. Pat. Nos. 5,554,775 and 6,034,027), azaborolinyl (U.S. Pat. No. 5,902,866), and indenoindolyl (U.S. Pat. No. 6,232,260; see also PCT Internat. Appl. WO 99/24446). Indenoindoles having a wide variety of substituent groups are easy to synthesize. Because substituent effects are readily exploited, the corresponding indenoindolyl complexes can be fine-tuned to achieve higher activity or to make polymers with a desired set of physical properties.
Single-site catalysts, including those containing indenoindolyl ligands, have traditionally been used with only a limited variety of activators. An important function of the activator is to generate a non-coordinating or weakly coordinating counterion for the cationic polymerization site. The activator helps in achieving an acceptable catalyst productivity. Usually, the activator is an alkyl alumoxane (e.g., methyl alumoxane), or an ionic borate or aluminate. (e.g., trityl tetrakis(pentafluorophenyl)borate). Unfortunately, alumoxanes are normally required in large excess, i.e., hundreds or even thousands of times the molar amount of the transition metal complex, which makes the catalyst system expensive. Ionic borates are sometimes more efficient than alumoxanes, but they are often costly to synthesize.
Recently, reaction products of organoboronic acids and alkylaluminum compounds (hereinafter sometimes called “aluminoboronates”) have been suggested as catalyst components for olefin polymerizations. For example, U.S. Pat. No. 5,414,180 teaches to react an alkyl- or arylboronic acid with a trialkylaluminum compound, and to use the reaction product in a metallocene-catalyzed olefin polymerization process. The metallocene complex used is typically bis(cyclopentadienyl)zirconium dichloride. In another example, U.S. Pat. No. 5,648,440 describes reaction products of organoboronic acids and trialkylaluminums and the use of these products as activators for bridged and unbridged metallocene complexes. This reference suggests that an advantage of these aluminoboronates is the ability to reduce the aluminum to transition metal mole ratio [Al:M] needed for satisfactory activity. Still unclear, however, is how well these aluminoboronate activators would perform with non-metallocene, single-site catalysts based on indenoindolyl ligands.
In sum, improved catalyst systems for polymerizing olefins are needed. Preferably, the catalyst systems would avoid the need for conventional activators, which are expensive to use. Ideally, the catalyst systems would take advantage of the flexibility of indenoindolyl ligand-containing complexes and would give polyolefins with a favorable balance of physical properties.